The present invention relates generally to controlling a memory in a video printer and in particular, to a method and circuitry for controlling the memory by adopting a frame line reading and writing system upon supplying printing data to a thermal print head of the video printer.
One of known arts for controlling a line memory providing a thermal print head (TPH) with printing data in a video printer is a field line system, a schematic block diagram of which system is disclosed in FIG. 2. Referring to this system, during a writing operation of printing data into a line memory 4, a controller 14 generates a given write address by controlling a write address counter 8, and a switch 10 transfers the write address from the write address counter to the line memory 14 under control of the controller 14. Hence, once analog video data from a video processor 1 are converted to digital data in an analog-to- digital converter 2 under control of the controller 14, the digital data are then supplied to the corresponding write address of the line memory 4 and stored therein as printing data for use in a subsequent printing operation of the video printer.
When reading the stored printing data out of the line memory 4 upon a printing operation, the switch 10 is connected to a read address counter 9, which under control of the controller 14 generates a set of read address to therefrom retrieve a corresponding one of the printing data previously stored in designated memory 4 allocations within the line memory, thereby transferring the read address to the line memory 4. Thus, a thermal print head (TPH) data processor 5 operably connected with the controller 14 and the line memory 4 receives the printing data read out of the line memory and, under control of the controller 14, provides the received data to a thermal print head (TPH) driver (not shown in the drawing) after processing a kind of conversion of printing data for better application in the TPH driver.
FIG. 1 illustrates the correlation between scanning of video data on a video screen and sampling of the video data to be stored into the line memory, by way of example, in view of the state of the art. Referring to this correlation, the video data on a single video screen are sampled and stored line by line into the line memory, and the stored video data are printed line by line, as follows: ##EQU1## wherein, n and m are respectively integer designations giving the maximum location of the video data scanning and sampling, and L.sub.1 through L.sub.m each designate a corresponding memory line in the line memory.
FIG. 3 is a schematic diagram for illustrating the correlation of consecutive read and write operations onto the line memory in the field line system, as shown in FIG. 2, provided with a repetition of one odd field and one even field of an input video signal. In this context of such a field line printing system every field of the input video signal is sequentially taken into the line memory by an interlacing scanning and sampling method. A common phenomenon of data collision occurs between odd fields such as a third field, a fifth field, a seventh field, etc., since the line memory is provided with only one input/output port in the usual cases. This data collision inevitably leads to poor picture quality in printing. Furthermore, while the line memory is usually adapted to store only video data corresponding to a single line to print out, it often fails to provide economical memory usage since nearly all commercially supplied memory devices do not fit such small sized line memory.
A U.S. Pat. No. 4,819,008 to Nagato et al. discloses a thermal print head driver circuit for a thermal printer permitting multiple graduation printing.